The present invention is directed to a color electrophotographic printer, and especially, the present invention relates to a color electrophotographic printer which can develop a toner image of four colors in two exposure times, or effect a simultaneous exposure of two spots.
A conventional color electrophotographic printer, as disclosed in Japanese patent Laid-open print No. 48-37148, employs a method wherein electrostatic latent images for both normal development and inversion development are formed on a photosensitive member, and wherein two colors of the latent images are developed in one exposure.
On the other hand, electrostatic latent images of four colors are formed in the prior art by two different exposures carried out sequentially, and then the formed electrostatic latent images are developed using four developing units. An example of this method of printing a color image is described in EP0582454A1 (Japanese patent Laid-open print No. 6-115174). As described in Japanese patent Laid-open print No. 6-115174, after the first charging and exposing operations, yellow color (inversion development) and black color (normal development) toner images are developed. After the second charging and exposing operations, the toner images of a magenta color (inversion development) and cyan color (normal development) are developed on the first-mentioned toner images.
In a conventional color electrophotographic printer, toners of a cyan color and a magenta color are physically superimposed on printing paper so as to form top and bottom layers, and the toners of the two colors are then melted and color mixed in the toner fixing unit. This technique of color mixing is referred to as subtraction color mixing and is an important technique to obtain a full-color image with color materials. However, in the apparatus disclosed in Japanese patent Laid-open print No. 6-115174, because toners of two different colors located in the same place on the photosensitive member can not be developed in one exposure, the technique of subtraction color mixing cannot be used.
In order to produce a blue color, color toners of a cyan color and magenta color are adjoined to each other in minute areas, and a high density is obtained by further mixing a black color in the toners. The method of adjoining toners in place is referred to as an additive color mixing method. In the case of the additive color mixing method, when the color materials like toners are color mixed, it is a well-known fact that the chromaticness, as well as the density, becomes small.
Next, because it is impossible to increase only the chromaticness, without increasing the density, by mixing in a black color, a blue color which is bright and has a high chromaticness can not be reproduced at all in a conventional color electrophotographic printer. For these reasons, because the color reproduction range of a record image is narrow, a full-color image with a high picture quality cannot be obtained. In Japanese patent Laid-open print No. 6-115174, it is indicated that the blue color produced by this color mixing method is merely approximated. In the apparatus described in this laid-open print, because two of the three primary colors must be developed by one exposure, at least one of the red, green and blue colors inevitably becomes an approximated color.